mask object spline points coordinates
Strangely enough, Blender calculates the relativ dimensions with respect to the larger dimension.
These words are not very well choosen. Here is a graphic that show a large orange square. Blender gives us relativ coordinates in this reference frame. The green rectangle is our movie aspect ratio.



import bpy

render = bpy.context.scene.render

coordinate = bpy.data.masks[0].layers[0].splines[0].points[0].co.copy()
coordinate.x *= render.resolution_x
coordinate.y = coordinate.y * render.resolution_x - (render.resolution_x - render.resolution_y) / 2

# at 100%, this gives us the correct coordinate
print(coordinate)